In recent years, an increasing number of engines are equipped with a cam phase alteration mechanism constituting a variable valvetrain which can alter the valve open/close timing (cam phase). Further, there has been developed a technique of using such cam phase alteration mechanism in an engine having plural valves for each cylinder to alter the open/close timing of only one or some of the plural valves, depending on operating conditions of the engine.
The camshaft used in such engine valvetrain is constructed by fitting separate cams to a shaft member in a manner allowing the cams to rotate relative to the shaft member. The cam phase alteration mechanism, for example in the form of a hydraulic actuator such as a vane actuator, is arranged at an end of the camshaft to alter the phase offset of some of the cams relative to the shaft member, thereby altering the split, or phase offset of some of the valves relative to the other valves (see patent document 1).